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12 WATER CONSERVATION THROUGH CRASSULACEAN ACID METABOLISM AND ITS SIGNIFICANCE TO AGRICULTURE Johns. Boyer, Department of Botany, University of Illinois Crassulacean acid metabolism (CAM) is a specialized metabolism found in some desert plants and epiphytes. All plants in the family Crassulaceae and many in the Bromeliaceae demonstrate this type of metabolism. Two examples of plants that exhibit CAM and are currently important as food for humans are pineapple and Opuntia. CAM is characterized by C02 uptake at night and C02 release during the day. The C02 is stored in malic acid and, during release, is in- corporated into photosynthetic products. The stomata are open during the night but generally are closed during the day; as a result, water loss by the plant is low. The average water loss from pineapple during 24 hours is 0.03 to 0.04 gmâ¢hr-1·100 cm-2 leaf area or about 1/25 to 1/10 that of most mesic crop plants. Average photosynthesis is 0.55 to 0.70 gm·hr-1·100 cm-2. Although pineapple has a low rate of photosynthesis as compared to mesic plants, unpublished data indicate that its dry matter production may be comparable to that of sugarcane. Opuntia is reported to have similarly high rates of growth under optimum condi- tions. The units of water required to produce a unit of dry matter in pineapple is SO, or one-fifth the lowest water requirement previ- ously reported for any plant. The possibility of high dry-matter production coupled with low water use may make pineapple and other similar plants important for raising the productivity of dry areas.
13 Root Zone and Drought Tolerance SOME BIOCHEMICAL ASPECTS OF ION UPTAKE BY ROOTS Euripedes Malavolta, Universidade de Sao Paulo, Sao Paulo, Brazil The main aspects of ion uptake, such as carrier concept, kinetics, and energy relationship, were described. The fundamental importance of such concepts for the understanding of plant nutrition was stressed. Some pertinent questions were raised concerning subjects that de- serve further study because of the basic nature of the applied impli- cations; energy coupling, nature of the carrier, and mechanisms of native species to thrive under conditions of soil-fertility stress.
14 HOW ROOTS ACQUIRE WATER FROM A SOIL PROFILE w. R. Gardner, Department of Soil Science, The University of Wisconsin Water uptake from the soil by plant roots is in response to a gradient of the water potential. Dehydration of plant leaves due to transpiration lowers the energy level of water in the plant and results in movement of water from soil into the plant largely by a passive pro- cess. This movement appears to be roughly proportional to the differ- ence between the water potential in the plant and that in the soil. In well-developed root systems the resistance to water movement in the soil does not appear to be significant when the water potential is above about -1.0 bar. As the soil becomes drier, the capillary conductivity of the soil decreases and eventually becomes limiting. The lower ex- tremities of some root systems studied in situ appear to offer con- siderable resistance to water transport-.- The relative importance of root extension and water movement through the soil with respect to rendering soil water available for uptake depends greatly upon the frequency and amount of water supplied to the soil profile.
15 SOIL CHEMICAL CONDITIONS AND PLANT ROOT DEVELOPMENT* Robert w. Pearson, u.s. Department of Agriculture, Auburn, Ala. Acidity and nutrient deficiencies or imbalances are probably the chemical conditions of the soil most likely to restrict plant root de- velopment in humid regions. Low soil pH inhibits roots by at least two avenues: (1) toxicity of the hydrogen ion itself, and (2) toxicity of aluminum, solubility of which increases with increasing soil acidity. Plant roots are generally believed to be fairly tolerant of the hydrogen ion per se within the limits of concentration commonly found in soils. Recent eVIdence, however, suggests that development of secondary root systems of cotton is retarded by the H ion at only moderate pH levels. The chief cause of inhibited root development in acid soils is the in- creased concentration of aluminum found in acid soil solution. Cotton root extension is retarded sharply at aluminum concentrations of only 0.2 to 0.3 ppm. Calcium deficiency is a potential hazard for root growth in acid soils, because of reduced amounts and because of the effect of aluminum in reducing calcium absorption by roots. In temperate zones, calcium deficiency is not a common cause of restricted root systems in acid soils but can be induced by heavy applications of water-soluble, noncal- cic fertilizers to poorly buffered acid soils. Root tolerance to acidity differs widely among plant species and even among varieties of the same species. Some grasses, such as napiergrass and bermudagrass, are extremely tolerant; others, such as sudangrass, are highly sensitive. Among the common crops of the South- eastern United States, cotton roots are very sensitive to low soil pH; soybeans are considerably less sensitive; and peanuts are able to de- velop normal root systems in very acid soils. Deep and extensive root systems are essential for crop use of sub- soil-stored moisture during periods between rains or irrigations. This requirement focuses attention on the importance of subsoil acidity. The extent and intensity of acidity below the plowed zone cannot be es- timated accurately for any area since subsoil samples are not routinely analyzed by soil testing laboratories, but subsoil pH levels of 4.5 to 5.0 are not uncommon in the Southeastern United States. Such strongly acid subsoils present an especially difficult problem because there is * Contribution from the Soil and Water Conservation Research Division, Agricultural Research Service, USDA, and the Alabama Agricultural Experiment Station, Auburn, Ala.
16 no practical way to mix lime below the plow depth, and downward movement of surface-applied lime is extremely slow. Intensive use of residually acid fertilizer aggravates this problem if an adequate liming program is not followed. However, when proper levels of lime are applied, high rates of residually acid fertilizer can be used without increasing sub- soil acidity. In fact, movement of bases into the subsoil can be ac- celerated by this practice.
17 SOIL PHYSICAL CONDITIONS AND ROOT DISTRIBUTION Lewis H. Stolzy, Department of Soils and Plant Nutrition, University of California Soil from a physical point of view is thought of as a three-phase system: solid, liquid, and gas. Soil variables are bulk density, water content, and air content, which are often expressed in terms of mass and volume or ratios of the three phases. The solid phase is re- ferred to as the matrix; it co;1trols the form, quantity, and distribu- tion of the other two. The three-phase soil system gives no expression of the many pro- cesses taking place in the soil. Most of these soil processes, in order to be described and related to root distribution, must be mea- sured separately for many soils under various conditions. One of the more dynamic physical aspects of many soils is the arrangement of the complex materials into similar repeating units caused by a complex pattern of forces, such as water movement, shrink- ing and swelling, freezing and thawing, biological pressures, and tillage. The shape, size, and stability of these units differ greatly, the extent of the differences depending on the composition of the matrix and the cementing agents holding the sands and clay together. Structural units in many soils determine to a large degree how water moves into and through the soil profile. The soil water in turn influences mechanical resistance, aeration, biological activity, and, to a lesser degree, soil temperature.
18 GEOGRAPHICAL DISTRIBUTION OF SOIL PROBLEMS RELATED TO LIMITATIONS IN ROOTING VOLUME Armand Van Warnbeke, Regional Soil Survey Office for Latin America, Food and Agriculture Organization, Santiago, Chile The horizons that reduce rooting volume of plants in soils can be grouped according to their origin. 1. Stone-lines occur less frequently in Latin America than in Africa. Although geological processes are important in their forma- tion, the activity of living organisms (ants and termites) should be taken into account in explaining the composition of soil profiles. No specific geographical zones of this kind of limitation can be rec- ognized on a South American soil map at small scale. 2. Cementation by silica, which leads to the formation of pans at varying depths in the soil, is mainly observed in recent volcanic ash sediments in Central America. These pans may restrict penetration by roots and reduce the permeability to water. Accumulation of iron hydroxides, as in plinthite, and their irreversible hardening into ironstone are correlated with fluctuating water tables in acid soils under wet-dry climates and with the subsequent retreat of groundwater. Plinthites are very common at tropical latitudes in the lowlands east of the Andes cordillera and in the wet land surfaces of the Amazon basin. 3. The distribution of argillic horizons in Latin America that may reduce rooting volume, as in planosols and solonetz, is governed by two factors. One is climatic; it requires seasonal variation in soil humidity. The other is pedological; it conditions the mobility of the clay. Salinity and alkalinity often induce illuviation of clay into strong B horizons, whereas acidity, 1:1 minerals, and Fe2o3 bindings frequently impede movement of clay as such. Therefore, argillic horizons are mostly located at high latitudes outside the areas that are underlain by the old continental shields where kaolinitic parent material predominates. 4. High water tables and associated gleys also limit the penetra- tion of the roots of most cultivated plants. 5. Mineral deficiencies reduce root growth, and lack of calcium is in many instances responsible for the absence of roots in soil ho- rizons that otherwise present favorable physical conditions. Acidity occurs mainly on the old continental sediments that are completely weathered and leached. They have generally been deforested and are under grassland vegetation, "Campo cerrado" soils of Brazil are typical examples of this kind of limitation.
